Wafer processing method

ABSTRACT

A wafer processing method for dividing a wafer having optical devices that are formed in a plurality of areas sectioned by dividing lines formed in a lattice pattern on the front surface, along the dividing lines, which comprises a laser beam application step of applying a laser beam to the wafer along the dividing lines from the side of the back surface thereof to form grooves having a predetermined depth in the back surface; a protective sheet affixing step of affixing a protective sheet to the front surface of the wafer having the grooves in the back surface; a dividing step of dividing the wafer having the protective sheet affixed to the front surface along the grooves; and a grinding step of grinding the back surface of the wafer divided along the grooves in a state of the protective sheet being affixed to the wafer, to remove the grooves.

FIELD OF THE INVENTION

The present invention relates to a wafer processing method for dividinga wafer having optical devices that are formed in a plurality of areassectioned by dividing lines (streets) formed in a lattice pattern on thefront surface, along the dividing lines.

DESCRIPTION OF THE PRIOR ART

A wafer having optical devices comprising a gallium nitride-basedcompound semiconductor which are formed in a plurality of areassectioned by dividing lines (streets) formed in a lattice pattern on thefront surface of a sapphire substrate and the like is divided along thedividing lines into individual optical devices such as light emittingdiodes or laser diodes which are widely used in electric equipment. Thiswafer is generally divided by a cutting machine called “dicer”. Thecutting machine comprises a chuck table for holding a workpiece such asa semiconductor wafer or optical device wafer, a cutting means forcutting the workpiece held on the chuck table, and a moving means formoving the chuck table and the cutting means relative to each other. Thecutting means comprises a rotary spindle that is rotated at a high speedand a cutting blade mounted to the spindle. The cutting blade comprisesa disk-like base and an annular cutting edge that is mounted on the sidewall outer peripheral portion of the base, and the cutting edge isformed as thick as about 20 μm by fixing, for example, diamond abrasivegrains having a diameter of about 3 μm to the base by electroforming.

Since a sapphire substrate, silicon carbide substrate, or the like has ahigh Mohs hardness, cutting with the above cutting blade is not alwayseasy. Since the cutting blade has a thickness of about 20 μm, thedividing lines for sectioning optical devices need to have a width ofabout 50 μm. Therefore, in the case of an optical device measuring, forexample, about 300 μm×300 μm, there is a problem that the area ratio ofthe dividing lines to the optical device is large, thereby reducingproductivity.

Meanwhile, a processing method for dividing a wafer by applying a laserbeam along streets is attempted and disclosed by JP-A 6-120334, forexample.

When a laser beam is applied to the front surface of a wafer to processit, however, heat energy is concentrated on the exposed area to producedebris that stick fast to the front surface of the wafer, therebygreatly reducing the quality of optical devices.

As means of dividing a wafer by applying a laser beam along streets, alaser beam processing method in which a laser beam capable of passingthrough the wafer is used and the laser beam is applied to the waferwith its focusing point on the inside of the area to be divided is alsoattempted and disclosed by JP-A 2002-192367, for example. In thedividing method using this laser beam processing technique, a workpieceis divided by applying a laser beam capable of passing through thewafer, for example, a laser beam having an infrared range, with itsfocusing point on the inside of the wafer from one side thereof tocontinuously form deteriorated layers in the inside of the wafer alongthe dividing lines and then, applying external force along the dividinglines whose strength has been reduced by the formation of thedeteriorated layers.

When the deteriorated layer is formed by applying a laser beam with itsfocusing point on the center portion of the inside of the area to bedivided of the wafer, there is a problem that it remains around theoptical device, and light emitted from the optical device is absorbed bythe deteriorated portion to reduce the brightness of the optical device.Further, when grooves are formed by applying a laser beam to the area tobe divided of the wafer and are cut, a molten layer remains on the sidesurfaces of the optical device, and light emitted from the opticaldevice is absorbed by the molten layer to reduce the brightness of theoptical device.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wafer processingmethod having high productivity and capable of dividing a wafer withoutreducing the brightness of an optical device.

To attain the above object, according to the present invention, there isprovided a wafer processing method for dividing a wafer having opticaldevices that are formed in a plurality of areas sectioned by dividinglines formed in a lattice pattern on the front surface, along thedividing lines, comprising:

a laser beam application step of applying a laser beam to the waferalong the dividing lines from the side of the back surface thereof toform grooves having a predetermined depth in the back surface;

a protective sheet affixing step of affixing a protective sheet to thefront surface of the wafer having the grooves in the back surface;

a dividing step of dividing the wafer having the protective sheetaffixed to the front surface along the grooves; and

a grinding step of grinding the back surface of the wafer divided alongthe grooves in a state of the protective sheet being affixed to thewafer, to remove the grooves.

Further, according to the present invention, there is provided a waferprocessing method for dividing a wafer having optical devices that areformed in a plurality of areas sectioned by dividing lines formed in alattice pattern on the front surface, along the dividing lines,comprising:

a protective sheet affixing step of affixing a protective sheet to thefront surface of the wafer;

a laser beam application step of applying a laser beam to the waferhaving the protective sheet affixed to the front surface thereof alongthe dividing lines from the side of the back surface thereof to formgrooves having a predetermined depth in the back surface;

a dividing step of dividing the wafer having the protective sheetaffixed to the front surface along the grooves; and

a grinding step of grinding the back surface of the wafer divided alongthe grooves in a state of the protective sheet being affixed to thewafer, to remove the grooves.

In the present invention, as the grooves formed along the dividing linesin the back surface of the wafer by the laser beam application step areremoved by carrying out the grinding step after the wafer has beendivided, the brightness of the optical devices is not reduced. In thepresent invention, since the wafer is cut along the grooves formed alongthe dividing lines, there is no cutting margin unlike cutting with acutting blade. Therefore, the area ratio of the dividing lines to theoptical device can be reduced and productivity can be thereby improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an optical device wafer to be divided bythe wafer processing method of the present invention;

FIGS. 2( a) and 2(b) are diagrams for explaining the laser beamapplication step in the wafer processing method of the presentinvention;

FIGS. 3( a) and 3(b) are diagrams for explaining the protective sheetaffixing step in the wafer processing method of the present invention;

FIGS. 4( a) and 4(b) are diagrams for explaining the dividing step inthe wafer processing method of the present invention; and

FIGS. 5( a) and 5(b) are diagrams for explaining the grinding step inthe wafer processing method of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A wafer processing method according to preferred embodiments of thepresent invention will be described in detail hereinunder with referenceto the accompanying drawings.

FIG. 1 is a perspective view of an optical device wafer 2 to be dividedaccording to the present invention. In the optical device wafer 2 shownin FIG. 1, a plurality of dividing lines 21 are formed in a latticepattern on the front surface 2 a thereof, and optical devices 22 inwhich a gallium nitride-based compound semiconductor and the like islaminated are formed in a plurality of areas sectioned by the dividinglines 21. This optical device wafer 2 has the optical devices 22measuring 0.3 mm×0.3 mm formed on the surface of a sapphire substratehaving a diameter of 2 inches and a thickness of 430 μm in theillustrated embodiment. The processing method for dividing this opticaldevice wafer 2 into individual optical devices 22 according to a firstembodiment will be described with reference to FIGS. 2 to 5.

In the first embodiment, the step of applying a laser beam to the aboveoptical device wafer 2 along the dividing lines 21 from the side of theback surface 2 b thereof to form grooves having a predetermined depth inthe back surface 2 b is first carried out. This laser beam applicationstep is carried out by a laser beam processing machine 3 as shown inFIG. 2( a). That is, the optical device wafer 2 is held on the chucktable 31 of the laser beam processing machine 3 in such a manner thatthe back surface 2 b faces up and positioned right below an imagepick-up means 32. Image processing such as pattern matching for aligninga laser beam application means 33 with the above dividing lines 21 iscarried out by the image pick-up means 32 and a control means (notshown) to perform the alignment of a laser beam application position. Onthis occasion, since the front surface 2 a having the dividing lines 21is located on the underside of the optical device wafer 2, the imagepick-up means 32 which is constituted by an infrared illuminating means,an optical system for capturing infrared radiation and an image pick-updevice (infrared CCD) for outputting an electric signal corresponding tothe infrared radiation is used to take an image of each dividing line21.

After the alignment of the laser beam application position is carriedout as described above, the chuck table 31 is moved to a laser beamapplication area where the laser beam application means 33 is locatedand processing-fed in a direction indicated by an arrow X while a laserbeam is applied from the laser beam application means 33 with itsfocusing point on the back surface 2 b, that is, on the top surface ofthe optical device wafer 2. The optical device wafer 2 is reciprocated25 times at a feed rate of 50 mm/sec. As a result, a groove 23 having,for example, a width of 50 μm and a depth of 200 μm is formed along thedividing line 21 in the back surface 2 b of the optical device wafer 2as shown in FIG. 2( b). The depth of the groove 23 must be set to avalue smaller than the finish thickness t (thickness from the frontsurface on which the optical device 22 is formed: for example, 200 μm)of the optical devices 22 formed on the optical device wafer 2,desirably 10 to 50% of the thickness of the optical device wafer 2. Thefollowing ultraviolet laser beam may be used as the laser beam appliedin the above laser beam application step.

-   Laser: YVO4 pulse laser-   Wavelength: 355 nm-   Pulse energy: 10 μJ-   Focusing spot diameter: 5 μm-   Pulse width: 12 ns-   Repetition frequency: 50 kHz

After the groove 23 is formed along the dividing line 21 in apredetermined direction formed on the optical device wafer 2 asdescribed above, the chuck table 31 or the laser beam application means33 is indexing-fed by the interval between adjacent dividing lines 21 inan indexing direction indicated by an arrow Y and processing-fed while alaser beam is applied again. After the above processing-feed andindexing-feed are carried out along all the dividing lines 21 formed inthe predetermined direction, the chuck table 31 is turned at 90° tocarry out the above processing-feed and indexing-feed along dividinglines 21 formed in a direction perpendicular to the above predetermineddirection, thereby making it possible to form grooves 23 along all thedividing lines 21 in the back surface 2 b of the optical device wafer 2.

After the above laser beam application step is carried out, theprotective sheet affixing step of affixing a protective sheet 4 to thefront surface 2 a of the optical device wafer 2 having the grooves 23formed in the back surface 2 b is carried out as shown in FIG. 3( a) andFIG. 3( b).

Thereafter, the dividing step of dividing the optical device wafer 2having the protective sheet 4 affixed to the front surface 2 a along thedividing lines 21 is carried out. In this dividing step, as shown inFIG. 4( a), the optical device wafer 2 is placed on a plurality ofcolumnar support members 5 arranged parallel to one another in such amanner that the back surface 2 b faces down. At this point, the opticaldevice wafer 2 is arranged such that the grooves 23 formed in the backsurface 2 b are each positioned between adjacent support members 5 and5. The protective sheet 4 affixed to the front surface 2 a of theoptical device wafer 2 is pressed along the grooves 23, that is, thedividing lines 21 by pressing members 6. As a result, a bending loadacts on the optical device wafer 2 along the grooves 23, that is, thedividing lines 21 to generate tensile stress in the back surface 2 b,thereby forming cracks 24 in the optical device wafer 2 along grooves23, that is, dividing lines 21, formed in a predetermined direction todivide the optical device wafer 2 as shown in FIG. 4( b). After theoptical device wafer 2 is divided along the grooves 23, that is, thedividing lines 21, formed in the predetermined direction, the opticaldevice wafer 2 is turned at 90° to carry out the above dividing workalong grooves 23, that is, dividing lines 21, formed in a directionperpendicular to the above predetermined direction, whereby the opticaldevice wafer 2 can be divided into individual optical devices 22. Sincethe individual optical devices 22 have the protective sheet 4 affixed tothe front surface 2 a, they do not fall apart and the shape of theoptical device wafer 2 is maintained.

After the dividing step is carried out as described above, the grindingstep of grinding the back surface 2 b of the optical device wafer 2 in astate of the protective sheet 4 being affixed to the front surface 2 aof the optical device wafer 2 to remove the grooves 23 is carried out.This grinding step is carried out by a grinding machine 7 comprising achuck table 71 and a grinding means 72 having a grinding whetstone 721as shown in FIG. 5( a). That is, the optical device wafer 2 is held onthe chuck table 71 in such a manner that the back surface 2 b faces upand the grinding whetstone 721 of the grinding means 72 is rotated at6,000 rpm and brought into contact with the back surface 2 b of theoptical device wafer 2 while the chuck table 71 is rotated at 300 rpm,to grind it. The optical device wafer 2 is divided into individualoptical devices 22 and ground to a predetermined finish thickness t (forexample, 200 μm) to remove the grooves 23 as shown in FIG. 5( b).

As the individual optical devices 22 from which the grooves 23 have thusbeen removed have no laser beam processed portion on the side surfaces,the brightness of the optical devices 22 does not lower. In the aboveembodiment, as the laser beam is applied to the back surface 2 b of theoptical device wafer 2 in the laser beam application step, even whendebris are produced, they do not adhere to the optical devices 22 formedon the front surface 2 a of the optical device wafer 2. The debris areremoved by grinding the back surface 2 b of the optical device wafer 2.

The processing method for dividing the optical device wafer 2 intoindividual optical devices 22 according to a second embodiment will bedescribed hereinbelow.

In the second embodiment, the order of the above laser beam applicationstep and the protective sheet affixing step in the first embodiment isreversed. That is, in the second embodiment, the step of affixing theprotective sheet 4 to the front surface 2 a of the optical device wafer2 is first carried out. Then, the optical device wafer 2 having theprotective sheet 4 affixed to the front surface 2 a is held on the chucktable 31 of the laser beam processing machine 3 in such a manner thatthe back surface 2 b faces up as shown in FIG. 2( a) to carry out theabove laser beam application step. After the protective sheet affixingstep and the laser beam application step are carried out, the dividingstep and the grinding step in the first embodiment are carried outlikewise in the second embodiment.

1. A wafer processing method for dividing a wafer having optical devicesthat are formed in a plurality of areas sectioned by dividing linesformed in a lattice pattern on a front surface, along the dividinglines, comprising: a laser beam application step of applying a laserbeam of a wavelength absorbed by the wafer to the wafer along thedividing lines from the side of a back surface thereof to form grooveshaving a predetermined depth in the back surface; a protective sheetaffixing step of affixing a protective sheet to the front surface of thewafer having the grooves in the back surface; a dividing step ofdividing the wafer having the protective sheet affixed to the frontsurface along the grooves; and a grinding step of grinding the backsurface of the wafer divided along the grooves in a state of theprotective sheet being affixed to the wafer, to remove the grooves. 2.The wafer processing method of claim 1, wherein the laser beamwavelength is 355 nm.
 3. A wafer processing method for dividing a waferhaving optical devices that are formed in a plurality of areas sectionedby dividing lines formed in a lattice pattern on a front surface, alongthe dividing lines, comprising: a protective sheet affixing step ofaffixing a protective sheet to the front surface of the wafer; a laserbeam application step of applying a laser beam of a wavelength absorbedby the wafer to the wafer having the protective sheet affixed to thefront surface thereof along the dividing lines from the side of a backsurface thereof to form grooves having a predetermined depth in the backsurface; a dividing step of dividing the wafer having the protectivesheet affixed to the front surface along the grooves; and a grindingstep of grinding the back surface of the wafer divided along the groovesin a state of the protective sheet being affixed to the wafer, to removethe grooves.
 4. The wafer processing method of claim 3, wherein thelaser beam wavelength is 355 nm.